Various non-limiting embodiments disclosed herein relate to photochromic materials having an extended pi-conjugated system. Other non-limiting embodiments relate to photochromic compositions and articles, such as optical elements, incorporating the same.
Many conventional photochromic materials, such as indeno-fused naphthopyrans, can undergo a transformation in response to certain wavelengths of electromagnetic radiation (or “actinic radiation”) from one form (or state) to another, with each form having a characteristic absorption spectrum. As used herein the term “actinic radiation” refers to electromagnetic radiation that is capable of causing a photochromic material to transform from one form or state to another. For example, many conventional photochromic materials are capable of transforming from a closed-form, corresponding to a “bleached” or “unactivated” state of the photochromic material, to an open-form, corresponding to a “colored” or “activated” state of the photochromic material, in response to actinic radiation, and reverting back to the closed-form in the absence of the actinic radiation in response to thermal energy. Photochromic compositions and articles that contain one or more photochromic materials, for example photochromic lenses for eyewear applications, may display clear and colored states that generally correspond to the states of the photochromic material(s) that they contain.
Typically, the amount of a photochromic material needed to achieve a desired optical effect when incorporated into a composition or article will depend, in part, on the amount of actinic radiation that the photochromic material absorbs on a per molecule basis. That is, the more actinic radiation that the photochromic material absorbs on a per molecule basis, the more likely (i.e., the higher the probability) the photochromic material will transform from the closed-form to the open-form. Photochromic compositions and articles that are made using photochromic materials having a relatively high molar absorption coefficient (or “extinction coefficient”) for actinic radiation may generally be used in lower concentrations than photochromic materials having lower molar absorption coefficients, while still achieving the desired optical effect.
For some applications, the amount of photochromic material that can be incorporated into the article may be limited due to the physical dimensions of the article. Accordingly, the use of conventional photochromic materials that have a relatively low molar absorption coefficient in such articles may be impractical because the amount photochromic material needed to achieve the desired optical effects cannot be physically accommodated in the article. Further, in other applications, the size or solubility of the photochromic material itself may limit the amount of the photochromic material that can be incorporated into the article. Additionally, since photochromic materials may be expensive, in still other applications, the amount of photochromic material be used may be limited due to economic considerations.
Accordingly, for some applications, it may be advantageous to develop photochromic materials that can display hyperchromic absorption of actinic radiation, which may enable the use of lower concentrations of the photochromic material while still achieving the desired optical effects. As used herein, the term “hyperchromic absorption” refers to an increase in the absorption of electromagnetic radiation by a photochromic material having an extended pi-conjugated system on a per molecule basis as compared to a comparable photochromic material that does not have an extended pi-conjugated system.
Additionally, as mentioned above, typically the transformation between the closed-form and the open-form requires that the photochromic material be exposed to certain wavelengths of electromagnetic radiation. For many conventional photochromic materials, the wavelengths of electromagnetic radiation that may cause this transformation typically range from 320 nanometers (“nm”) to 390 nm. Accordingly, conventional photochromic materials may not be optimal for use in applications that are shielded from a substantial amount of electromagnetic radiation in the range of 320 nm to 390 nm. For example, lenses for eyewear applications that are made using conventional photochromic materials may not reach their fully-colored state when used in an automobile. This is because a large portion of electromagnetic radiation in the range of 320 nm to 390 nm can be absorbed by the windshield of the automobile before it can be absorbed by the photochromic material(s) in the lenses. Therefore, for some applications, it may be advantageous to develop photochromic materials that can have a closed-form absorption spectrum for electromagnetic radiation that is shifted to longer wavelengths, that is “bathochromically shifted.” As used herein the term “closed-form absorption spectrum” refers to the absorption spectrum of the photochromic material in the closed-form or unactivated state. For example, in applications involving behind the windshield use of photochromic materials, it may be advantageous if the closed-form absorption spectrum of the photochromic material were shifted such that the photochromic material may absorb sufficient electromagnetic radiation having a wavelength greater than 390 nm to permit the photochromic material to transform from the closed-form to an open-form.